For this, as regards refractive beam shapers, various optical systems are known in the state of the art. They usually use at least two lenses and are based on the principle of a Keplerian or Galilean telescope.
Thus, for example, in EP 0 240 194 A1 an afocal beam expander is described which is constructed from a negative lens element and a positive lens element which are spaced apart from each other. The two lens elements consist of different materials which are selected such that temperature-related changes in the refractive indices of the two elements are compensated for. Different lens pairs are described for different changes in the diameter of the beam, the curvatures of the lens surfaces can be spherical or aspherical. Different lens combinations must thus be used for different magnifications, and a renewed adjustment must take place after each change.
In DE 691 06 569 T2 an achromatic zoom beam expander is described. This consists of three individually color-corrected component pairs, wherein one component pair is arranged fixed and the other two component pairs can be moved relative to the fixed component pair and relative to each other. An adjustable magnification can be realized in this way, this is a beam expander of the Galilean telescope type, which expands a collimated laser beam to a, likewise collimated, laser beam with a larger cross-section.
With the device described in DE 691 06 569 T2, although different magnifications can be variably set, this system requires three lens groups of two lenses cemented together each, thus a total of six lenses. For one thing, this makes the structure complicated and, for another, because of the number of at least six lenses, it is more costly than the solution described in the unexamined European patent application EP 0 240 194 A1, in which, for example, one of the two lenses is changed to set another magnification. In addition, the solution described in DE 691 06 569 T2 is not suitable for use with those lasers that have a high power output. This could result in damage to the lens group, as the cementing of the individual lenses to form a lens group reacts more sensitively than the glass of the lenses; but also glasses no longer withstand particularly high powers, with the result that in this case a switch is made to catadioptric systems.